1. Field of the Invention
The present invention generally relates to rotary blood pumps that can be implanted into the chest of humans and can be used to assist a human heart in pumping blood, and, more specifically, to such blood pumps that use magnetic suspensions.
2. Description of the Prior Art.
The implantable blood pumps according to the latest technology that are now being developed to assist the heart are turbo pumps. They come in axial flow configurations, such as the Jarvik 2000; centrifugal configurations, such as one being developed by the Cleveland Clinic; and mixed flow types such as the xe2x80x9cStreamlinerxe2x80x9d being developed at the University of Pittsburgh. All employ a high-speed rotary impeller rotating at thousands of rpm. Most, including the Jarvik 2000, use hard-contact journal bearings to support the rotor. Such use is not desirable because blood damage and thrombosis can be caused by the bearings. To try to circumvent contact-bearing problems, magnetic bearings are now being employed, as in the xe2x80x9cStreamlinerxe2x80x9d pump. These are non-contacting bearings and result in minimal blood damage, since the bearing clearances can be kept large to reduce shear stress in the blood. However, the problem still exists of having to thoroughly wash out all the bearing clearances with fresh blood. This washout is essential to eliminate the formation of thrombus.
Magnetic bearings must be packaged in a small space in order to minimize the size of the pump, and this can be quite difficult. Most magnetic bearing pumps are too large and therefore unacceptable.
Another requirement for implantable blood pumps is low power consumption. Pumps that employ magnetic bearings are notorious for their power consumption, which can be-as high as 20 watts for just the bearings, 5 watts for the bearing(s) being more typical. The power delivered to the blood in a left ventricular assist device (LVAD)is about 3.0 wafts, so one does not want to expend more than 1.0 additional watt for the magnetic bearings.
Most magnetic bearings use permanent magnets or electromagnets to-generate radial magnetic fields that directly suspend the rotor radially. However, the bearing radial xe2x80x9cstiffnessxe2x80x9d obtained using the relatively low air gap fields produced by the magnets is not high. A large bearing is, therefore, needed to hold imposed loads with small radial deflection.
Radially passive magnetic bearings are inherently unstable axially, as stated by Ernshaw""s Law. Active axial control is, therefore, required to stabilize a rotor suspended by such bearings. Particularly for an axial flow turbo pump that has substantial axial forces acting on the rotor, the power consumed by the active coils can be unacceptably large. A xe2x80x9cvirtually zero powerxe2x80x9d (VZP) control loop is sometimes used to reduce power consumption. This control is generally known as VZP control and was first used back in the 1970s by J. Lyman, one of the founders of magnetic suspensions.
Implantable turbo blood pumps are typically run at constant rpm because it has been difficult to close the loop around the patient and physiologically vary pump flow rate according to the needs of the patient. By providing a base rate of flow, increased blood demand due to activity level is made up by the natural heart. However, a sick heart cannot make up much demand, and activity level is limited. Whatever cardiac output demand is made up by the patient""s heart undesirably loads the sick left ventricle. To physiologically control pump output flow, extraneous sensors have sometimes been added to measure physiologic parameters of the patient. These have included the addition of blood pressure transducers to measure the pump outlet pressure or differential pressure. This is highly undesirable because the addition of extraneous sensors can cause thrombosis and long-term hemodynamic reliability concerns. A known LVAD uses an invasively placed series ultrasonic flowmeter to determine pump flow rate since the LVAD cannot directly measure its output blood pressure.
The natural heart produces pulsatile flow. Experiments have shown that this unsteady flow minimizes the onset of thrombosis in the larger arteries of the body because the flow pattern constantly changes. In a pulsatile flow pump, areas of stagnant flow are minimized or eliminated not only in the patient""s arteries at the pump outlet, but within the pump itself. Current turbo pumps are direct current (DC) or steady flow devices that do not produce pulsatile flow. Even as the heart of a sick patient recovers and contributes some degree of pulsatile flow to the body, the degree of pulsatility is much less than that of the natural heart since the LVAD blood pump is unloading the sick heart. For xe2x80x9cBridge To Recoveryxe2x80x9d long-term implants, pulsatile flow from the LVAD is highly desirable.
Accordingly, it is an object of this invention.to provide alternate means to wash out the magnetic bearing gaps with fresh blood to eliminate thrombus formation at the bearings.
It is another object of the invention to allow bearing washout under minimal flow conditions through the pump.
It is still another object of the present invention to provide non-contact active washout means for the bearings.
It is yet another object of the present invention to provide a magnetic bearing geometry that is easily washed out by the blood flow to prevent areas of stasis.
A further object of this invention is to provide a small size bearing system that is simple in construction and packageable with the various turbo pump types for use with both adults and children.
A still further object of the present invention is to provide a control system that requires very low power when used with the disclosed high load capacity bearings.
It is yet a further object of the present invention to determine pump differential pressure in a direct manner without the addition of extraneous sensors.
It is an additional object of this invention to provide an active coil and magnet geometry that requires low power approaching zero to sustain axial loads.
It is still an additional object of the invention to provide safety of pulsatile flow by eliminating the undesirable condition of reverse flow through the pump.
It is yet an additional object of this invention to provide pulsatile flow in a reliable manner using pump differential pressure determined directly by the magnetic bearings.
It is also an object of this invention to shorten the length of an Archimedes screw type axial flow impeller by providing multiple parallel flow blades that minimally overlap. In mini-size blood pumps, for which this invention is intended, minimizing axial length of the pump is desirable particularly for applications in small women and children.
It is furthermore an object of this invention to provide a compact outlet stator of short axial length that does not damage blood while recovering impeller pressure.
In order to achieve the above objects, as well as others that will become evident hereafter, a blood pump in accordance with the present invention comprises a pump housing defining a pump axis, and inlet and outlet openings at opposite axial ends of said pump housing. A rotor is provided that defines a rotor axis and opposing rotor axial ends. Magnetic suspension means is provided within said pump housing at said rotor axial ends for magnetically suspending said rotor and passively maintaining the radial stability of said rotor so that said rotor axis remains substantially coextensive within said pump axis during operation. Control means is provided for maintaining axial stability of said rotor so that said rotor may absorb externally imposed axial loads and so that contact of said rotor within said pump housing is eliminated. Impeller means on said rotor operates to draw blood into said inlet opening and expel the blood through said outlet opening with rotation of said rotor. Drive means is provided for rotating said rotor and impeller means to thereby pump the blood, fluid gaps being formed between said rotor axial ends and said magnetic suspension means. Blood washout means is provided for continuously moving blood through said fluid gaps during rotation of said rotor to prevent formation of thrombus in said fluid gaps.
Preferably, the washout means provides positive or active flow of blood through fluid gaps where stagnation of blood might otherwise take place. In accordance with another feature of the invention, the drive means is arranged to drive the impeller means at a selected rotational speed, and means are provided for sensing the pressure differential within said pump housing and imparting a cyclic variation to said selected rotational speed of said drive means to provide pulsating movements of the blood through the pump and into the patient""s circulatory system. Also, one structure for washout relies on generating differential pressures across the bearing gaps in a passive manner using the flow itself. An alternate structure attaches Archimedes screw pumps to the front and rear of the rotor to actively pump blood through the bearing gaps. In approximately 30% of heart-assist patients the natural heart re-conditions sufficiently after a year or two of LVAD use so that the pump is no longer needed. Rather than explant the device, the pump can be left in place and operated at minimal flow and power consumption. The active screw pumps allow proper bearing gap washout when the pump is put to sleep and minimally used.
An important feature of the invention is to mount the impeller means on a magnetically suspended rotor that is inherently stable in radial directions and to provide direct feedback signals useful for stabilizing the rotor in the axial direction. This feature substantially simplifies the design and construction of the pump, reduces its cost of manufacture and substantially enhances the reliability over extended periods of use.
A compact high radial stiffness magnetic bearing uses axial fringing ring magnetic fields to passively support the pump rotor radially. The flux is focused or concentrated from a permanent magnet in the fringing rings to produce very high radial load capacity in a small size. This is different than typical radially passive magnetic suspensions that employ radial magnetic fields. Active axial control stabilizes the bearing using a xe2x80x9cVirtually Zero Powerxe2x80x9d control feedback loop. Low power and small size make the bearing applicable to axial flow and other configuration blood pumps particularly suitable for implantation. Differential pressure across the bearing fluid gaps, forcefully positively or actively washes the gaps with fresh blood to eliminate thrombus and flow stagnation. The rotor force on the magnetic bearings can be measured by the bearing control system. This allows the direct determination of differential pressure across the pump. This parameter can be used to obtain a pulsatile output pressure and flow and to exert physiological control on the pump output so as to match the patient""s activity.
In the present invention; a very high radial stiffness bearing is obtained. This is accomplished by employing an axially directed fringing ring field that has a radial load capacity an order of magnitude higher than radially directed fields. This allows one to use a small diameter bearing that was not heretofore feasible. The high-load capacity results in low power consumption as well.
Until the present invention, it has not been possible to determine turbo pump differential pressure in a direct manner. Turbo pump differential pressure can be used in part to exert physiological control on the pump flow rate as demanded by the patient""s activity level and heart rate.